The present invention relates to an image storage system for storing image data of a 2.sup.N .times.2.sup.N matrix size in a storage medium, and reading out the image data therefrom, in order to display it, and, more particularly, to an image storage system which is effective for improving the efficiency of retrieving the image data from the storage medium.
Storage and retention of image data are often necessary. However, when image data which is stored in a storage medium is read out and displayed, as a matrix size of original image data increases, the time required for transferring image data of one frame to be visualized on a display device is prolonged. When, for example, medical image data is read out from a storage medium and is displayed for diagnosis, it may take a long time in a process of image data transfer and visualization, degrading diagnosis efficiency. In particular, recently, image data constituted by gradation data, e.g., gray-scale data for each pixel having a great data length, e.g., about 16 bits, is used. Use of image data consisting of gradation data having a great data length results in a prolongation of the time required for transferring and visualizing an image of one frame. For this reason, when a plurality of frames of image data are stored in a storage medium, a very long time is necessary to retrieve a desired image from the medium and display it.
In order to shorten a time required for transfer/visualization and to improve retrieval efficiency, a data companding technique is employed such that image data is compresssed and is transferred and visualized. In the data companding technique that can be applied to image data, at most 30% data compression can only be performed if compression/expansion (restoration) is performed without losing a data volume of the image data. If data compression at a higher ratio is performed, image data may be partially lost.
As described above, in the data companding technique, an image data compression ratio free from data loss is at most 30%, and a time required for transfer/retrieval can only be shortened accordingly. Therefore, in a conventional system, it is difficult to greatly improve image data transfer efficiency and to greatly shorten a time required for retrieval.